1. Technical Field
An embodiment of the present invention relates generally to medical diagnostic imaging such as Single Photon Emission Computed Tomography (SPECT) nuclear medicine studies and correction of data attenuation in such studies by using data acquired from a transmission source to generate an attenuation correction map. More specifically, an embodiment of the invention relates to extension or extrapolation of image data from a transmission scan beyond the limited field of view (FOV) of a gamma-ray detector so as to extend the transmission projection to a larger field of view corresponding to the actual area of a patient. The embodiment of the invention thus compensates for truncation of the transmission data by the limited FOV, to enable reconstruction of transmission projection data free from artifacts caused by truncation.
2. Background
In nuclear medicine imaging techniques such as SPECT and Positron Emission Tomography (PET), medical images are regenerated based on radioactive emission data, typically in the form of gamma rays, emitted from the body of a patient after the patient has ingested or been injected with a radiopharmaceutical substance. Emitted gamma rays are detected from numerous different projection angles by a gamma camera (a.k.a. Anger camera or scintillation camera) about a longitudinal axis of the patient, and converted into electrical signals that are stored as image data. Data from image projections provide a set of images as a result of a process known as image reconstruction.
In a conventional SPECT study of an organ such as the heart, a radioisotope (Tc-99m, TI-201, for example) is administered to the patient and the radioisotope is taken up by the heart muscles. Then, the patient is placed in an imaging bed of a scintillation camera system and one or more scintillation camera detectors are rotated about the long axis of the patient and interact with gamma emissions from the patient's body at various angular orientations about the axis. The resulting data is used to form three-dimensional images (known as “SPECT images” or “tomographic images”) of the distribution of the radioisotope within the patient.
Such three-dimensional SPECT images can be calculated based on a set of two-dimensional images (“projections” or “projection images”) acquired by the scintillation camera system as the detectors are rotated about the patient in a series of steps; this calculation process is known as image reconstruction. The most commonly employed method of image reconstruction is known as filtered back-projection or FBP. When FBP reconstruction is used to reconstruct SPECT images from two-dimensional projection images obtained from a scintillation camera, some well-recognized distortions introduce errors or artifacts in the result. One of the most critical distortions is caused by attenuation of gamma radiation in tissue. As a consequence of attenuation, quantitative image values in the various projections do not accurately represent line integrals of the radioisotope distribution within the body. It is therefore necessary to correct for this distortion, and the process for doing so in SPECT is known as attenuation correction.
It is known to measure the actual attenuation coefficients of body tissues by placing a line source of gamma radiation on one side of the body and measuring the transmission of the gamma radiation through the body as a function of direction, i.e. collecting transmission CT data, as the line source is scanned across the patient's body. See, e.g. U.S. Pat. No. 5,576,545 (Stoub et al.) incorporated herein by reference in its entirety.
For small FOV cameras, such as the SIEMENS c.cam cardiac SPECT camera, the width of a patient can be much larger than the FOV, and therefore the transmission image projections at various projection angles are truncated, causing unwanted artifacts in transmission reconstruction, and thereby resulting in inaccurate attenuation correction maps.
Truncation compensation in transmission reconstructions for small field of view (FOV) cardiac SPECT cameras using arrays of line sources has not been previously addressed in the prior art. Attenuation correction using transmission data obtained from moving line sources is offered both by General Electric (Millennium ACuscan for the Millennium MG and MyoSIGHT systems) and Philips Medical Systems (VantagePro AC add-on for the CardioMD). However, only the VantagePro add-on offers truncation compensation.
The problem of truncation in transmission image reconstruction has been widely studied for SPECT imaging in general. In some of these reconstruction methods, the transmission reconstructions were extended using various data extrapolation methods. (See, Tsui, BMW, et al., “Cardiac SPECT reconstructions with truncated projections in different SPECT system designs,” J. Nucl. Med., 1992, 33:5, 831.) Other approaches include using singular value decompositions (See, Zeng G L, Gullberg G T, “An SVD study of truncated transmission data in SPECT,” IEEE Trans. Nucl. Sci, 1997, 44:1, 107-111.) or knowledge of sets of know cross-sections. (See, Panin V Y, Zeng G L, Gullberg G T, “Reconstructions of Truncated Projections using an optimal basis expansion derived from the cross-correlation of a knowledge set of a priori cross-sections,” IEEE Trans. Nucl. Sci, 1998, 54:4, 1229-2125.)
Reconstruction of truncated projections has also been investigated for CT images and for PET/CT systems that use CT images for attenuation correction. These techniques involve extension of the transmission projections beyond the FOV using a variety of extrapolation techniques with different constraints. (See Ohnesorge et al., “Efficient Correction for CT Image Artifacts Caused By Objects Extending Outside Scan Field of View,” Med. Phys., 27:1, 39-46, January 2000.)